We know very little about pollination in the Platanthera Rich (Orchidaceae: Orchidoideae)

Abstract The Platanthera Rich. (Orchidoideae) comprise a speciose genus of orchids primarily in the northern hemisphere, with up to 200 known species worldwide. Individual species are known to self‐pollinate, but many rely on insect pollinators with characteristics such as floral color, timing of floral odor emissions, nectar rewards, and spur length associated with particular pollination syndromes. As with many orchids, some orchid–pollinator associations are likely highly co‐evolved, but we also know that some Platanthera spp. are the result of hybridization events, which implies a lack of pollinator fidelity in some cases. Some Platanthera spp. occur in large numbers which, coupled with the numerous Platanthera–pollinator systems, make them accessible as study species and useful for co‐evolutionary studies. Due to the likely effects of climate change and ongoing development on Platanthera spp. habitats, these orchids and their associated pollinators should be a focus of conservation attention and management. However, while there is a fairly substantial literature coverage of Platanthera–pollinator occurrence and interactions, there are still wide gaps in our understanding of the species involved in these systems. In this systematic review, we outline what is current knowledge and provide guidance on further research that will increase our understanding of orchid–insect co‐evolutionary relationships. Our review covers 157 orchid species and about 233 pollinator species interacting with 30 Platanthera spp. We provide analyses on aspects of these interactions such as flower morphology, known insect partners of Platanthera species, insect‐Platanthera specificity, pollination visitor timing (diurnal vs. nocturnal), floral rewards, and insect behavior affecting pollination outcomes (e.g., pollinia placement). A substantial number of Platanthera spp. and at least a few of their known pollinators are of official (IUCN) conservation concern – and many of their pollinators remain unassessed or even currently unknown – which adds to the urgency of further research on these co‐evolved relationships.


| INTRODUC TI ON
Pollination services are crucial for the reproduction of most angiosperms.Over 87% of angiosperms are believed to be pollinated by animal vectors (Chupp et al., 2015), and it is estimated that we know of pairings between approximately 400,000 insect and 350,000 plant species (Schatz et al., 2020).Animal-provisioned pollination can be more efficient and more specific than abiotic pollination (e.g., wind pollination), resulting in increased seed set, and contributes to floral diversification, variable gene flow, species isolation or co-existence, and speciation.Thus, pollination ecology is a key area of study in plant-animal interaction research, with many models, hypotheses, and categorizations of interaction types.
A pollination syndrome is a particular suite of floral traits that adapts a flower for a specific group of pollinators or even a single, specialized pollinator.Thus, floral traits should adapt to the most efficient pollinator (Stebbins, 1970), driving convergent evolution of floral features in many plant species (Dellinger, 2020).Typically, syndromes will reflect floral trait evolution in response to a functional group of pollinators (e.g., moth, fly, and bird), and syndromes have often been used to predict pollinator groups (Ollerton et al., 2009).For example, white flowers with a nectar spur that produce scent at night might be expected to attract a nocturnal moth pollinator through a reward system.In fact, Charles Darwin made exactly this prediction when examining the long nectar spurs of the orchid (Orchidaceae) Angraecum sesquipedale (Arditti et al., 2012); the prediction was proven true with the later discovery of the sphinx moth, Xanthopan morganii praedicta (Arditti et al., 2012).In spite of widespread use as a research concept, pollination syndromes have been criticized for a lack of consistency in the floral traits used, issues with terminology, definitions of what constitutes a pollinator, and for oversimplification of complex interactions (Dellinger, 2020;Fenster et al., 2004;Ollerton et al., 2018).
Pollination syndromes provide a mechanistic link to help explain morphological variation arising from differences in male and female reproductive fitness and convergent evolution of floral traits when relying on similar pollinator groups.The Bateman principle (Arnold, 1994) suggests that female fitness is limited by ovule or seed production and that male fitness is limited by the ability to fertilize ovules (Catling & Catling, 1991;Chupp et al., 2015).
Thus, floral traits that maximize pollen transfer should be under stronger selection pressure than traits that maximize pollen deposition and subsequent fertilization rates (Arnold, 1994).Applying the Bateman principle is challenging in plants because pollinators are often limited, observations of pollination and subsequent fertilization can be rare, and because of the sessile nature of plants, high rates of polyandry are assumed to be common (Tonnabel et al., 2019).
In spite of high species richness, and some authors suggesting that orchids are over-represented in the pollination literature compared to other plant groups (Peakall, 2007), there is still a great deal we do not know about orchid pollination (Schatz et al., 2020) and its role in the continued evolution of orchid-insect interactions.
Given the diversity in orchid pollination, orchid-insect interactions represent useful models for understanding pollination ecology in other plant groups.In addition, high numbers of orchid species are at risk of extinction (Swarts & Dixon, 2009), there is widespread concern that pollinator populations are declining (Owens & Lewis, 2018;Young et al., 2017), and that climate change will lead to range shifts (Bedford et al., 2012;Gómez-Ruiz & Lacher, 2019) and phenological asynchronies (Adedoja et al., 2020;Slominski & Burkle, 2021) that will further impact plant-insect interactions among the Orchidaceae and beyond.Thus, understanding orchidinsect relationships is increasingly important not only for orchid conservation but also for developing and applying an understanding of pollination networks and biodiversity conservation in a variety of plant communities.
exhibit diverse floral morphology and several pollination syndromes have been characterized for the genus (Argue, 2012;Hapeman & Inoue, 1997).The diversity of floral features, coupled with the frequent reports of hybridization among certain species (e.g., P. bifolia × P. chlorantha (Esposito et al., 2017), and P. dilatata × P. aquilonis (Wallace, 2004)), make Platanthera spp.an excellent study system for advancing our understanding of pollination ecology, the application of pollination syndromes, plant-insect interactions, reproductive

T A X O N O M Y C L A S S I F I C A T I O N
Biodiversity ecology, Botany, Entomology, Evolutionary ecology, Population ecology barriers, floral evolution, and general patterns of orchid and pollinator diversity.However, our advances in these areas are often hampered by orchid species delineations, which rely on some of the same pollination syndrome characters that are subject to pollinatordriven selection pressures (Hapeman & Inoue, 1997), a lack of natural historical observations, and the need for targeted research to distinguish non-pollinating floral visitors from pollinators.
Because it is a widespread, species-rich group that seems particularly amenable to ecological and evolutionary research questions, we were interested in understanding the current state of the Platanthera pollinator literature, with a particular focus on the accuracy of currently applied pollination syndromes, and what knowledge gaps remain.Reviews by van der Cingel (2001), Argue (2012), and Pace (2020) each covered a subset of the species present within the genus because they focused on different geographic regions and highlighted different aspects of pollination.The review by Pace (2020) focused on systematics of orchids in northeastern North America and only superficially mentioned pollination ecology.
Whereas the reviews by van der Cingel (2001) and Argue (2012) presented broad overviews of pollination systems (e.g., rewarding or not; pollination syndromes) but lacked specific detail regarding pollinator species, behavior, floral trait variation, and reproductive success.Thus, a comprehensive review specifically of the Platanthera, and their pollinators, across their geographic range has not been conducted.This review spans all of the current, accepted, reported species from across the entire geographic range, which includes every continent except Antarctica and South America.

| Selection of species and sources of information
Our approach consisted of five steps (see Figure 1).First, we obtained a complete list of commonly accepted species within the Platanthera from The Plant List on 10th May 2022 (http:// www.thepl antli st.org/ tpl1.1/record/ kew-157023).The accepted names were used to focus our readings and data compilation as orchid taxonomy is quite dynamic (Chase et al., 2015;Dressler, 1993;Janes & Duretto, 2010).
We included Piperia spp.(the rein orchids) in our search terms as the genus was not officially amalgamated with the Platanthera until 2009 (Bateman et al., 2009).Third, we performed a complimentary literature search, using Publish or Perish v7.33.3388 (Harzing, 2007), that cross-referenced Google Scholar.Given the number of records that Google Scholar searches can generate and the dynamic nature of its search algorithms (Giustini & Boulos, 2013), we restricted our search dates to 2001-2022 across titles and abstracts, to account for the fact that one of the last reviews of Platanthera spp.
While Argue (2012) did not appear to add substantial new information for the Platanthera, and often cited van der Cingel (2001).
Fourth, we manually filtered the combined results from Web of Science and Google Scholar.These filters included: de-duplicated records within database searches, removing Web of Science records prior to 2001, de-duplicating records across database searches, removing non-English articles for which a translation could not be found, and removing uninformative records based on title and abstract.For example, several papers describing complete chloroplast genomes for Platanthera spp.did not contain useful pollinator or floral visitor information.Fifth, because many sources cited others for their information, we obtained as many of the original articles as possible using web sources or institutional interlibrary loans where necessary to verify the information.

| Data collection and categories
While collecting information on Platanthera-insect associations, we collected data on Platanthera morphology, distribution, scent, and reproductive success when available in the literature.Full details of data collected can be found in Appendix S1.Given the inconsistency of information provided across sources, we applied broad categories to some elements of the data.For example, orchid species' distributions were determined at a continental scale.Platanthera spp.were grouped according to descriptions of several floral traits believed to be useful in characterizing pollinator syndromes and explaining reproductive isolation among sympatric and/or sister taxa (Boberg et al., 2014;Brand et al., 2020;Schiestl & Schlüter, 2009) -flower color, spur length, labellum type, and scent.Morphological categories were determined based on common language or characters provided by authors.For example, many authors applied the term 'short' to spurs measuring 10 mm or less.While such categories are broad, and somewhat subjective, similar approaches have been used by others (Hapeman & Inoue, 1997;Schiestl & Schlüter, 2009), and provided insight for our categorization.

| Statistical analysis and data visualization
Means were calculated for a number of categories including insect taxa per orchid species, and orchid species.A pollinator sharing index (Schiestl & Schlüter, 2009) was calculated for each pollinator species (index = (orchid spp.sharing the pollinator -1)/(total number orchid spp.-1)).Index values range between 0 and 1, with 0 indicating no pollinator sharing and 1 indicating complete overlap in pollinators.A two-tailed Fisher Exact test was performed in Microsoft Excel to assess independence among flower color and insect activity time.Venn diagrams were made in R 4.2.2 (R Core Team, 2021) using packages ggvenn (Yan, 2021) and ggVennDiagram (Gao et al., 2021).

Appendix S1
).We sourced and read the remaining 246 records; 34.62% (71) literature sources were useful in contributing pollinator or floral visitor information.An additional 51 papers were sourced and read because they were cited by others, bringing the total number of unique literature sources to 297.Thus, 122 (41.08%) of these 297 literature sources contributed to this review.Refer to Appendix S1 for full details of literature sources that contributed to this review.We were unable to acquire full-text copies of some literature sources cited by others.Similarly, we were unable to verify some information sourced from personal communications.These instances are highlighted in Appendix S1.

| General Platanthera information and trends
The Plant List returned 157 accepted Platanthera spp., of which 18 (11.46%)are recognized hybrids, and 20 (12.74%) have been assessed for an IUCN status (Table 1).One species, P. clavellata, was cited as being definitively autogamous or lacking any pollinator (Catling, 1983).In terms of continental distribution, 64 (40.76%) species were reported for Asia, 47 (29.94%) for North America, 10 (6.37%) for Europe, five (3.18%) for Australia, and two (1.27%) for F I G U R E 1 Workflow of search and filter steps to obtain literature records used in this review.

TA B L E 1
Accepted Platanthera species with their respective distributions, morphologies, and ecological information.

TA B L E 1 (Continued)
Africa (Figure 2).No species were reported in the literature from South America.While there are records of Platanthera spp.from South America on iNaturalist, it is possible that these are misidentifications.A striking 41 species (26.11%) are missing distribution information based on the sources of information we read.
Table 1 provides detailed morphological information for a number of Platanthera spp.In total, 80 (50.96%)Platanthera spp.were described as being green, 44 (28.03%) were described as white, while 48 (30.57%) were described as other colors, such as purple or orange, while 43 (27.39%)species did not have a color description.
Many species were placed in multiple categories due to descriptions of greenish-white or white-green, for example (Figure 3a).The labella of 83 (52.47%) species were described as entire, whereas 22 (14.01%)species were described as having fringed or dissected labella.Fringed labella were only described for species that occur in North America.There were 52 (33.12%) species without a labellum description.
Using broad categories of spur length (short 1-10 mm, medium 11-20 mm, long 21-30 mm, and very long >31 mm), we found considerable variation among reported lengths for various species, with many species placed in multiple categories.For example, 62 (39.49%) species had small spurs, 49 (31.21%) had medium spurs, 43 (27.39%) had long spurs, and seven (4.46%) had very long spurs (Figure 3c).We could not find definitive measurements for 65 (41.40%)species.Three species (1.91%) were described as having no scent production, 27 (17.20%)were described as emitting a human-detectable scent, and 127 (80.89%) species had no scent information provided.Among those with a detectable scent, 11 (40.74% of scented species) were described as releasing the scent at night, five (18.52% of scented species) release the scent during the day, and three (11.11% of the scented species) were cited as releasing the scent both day and night.One species (Platanthera hyperborea) was described as being scented by one author and non-scented by another but this may be attributed to changes in taxonomy and known distributions of species -both P. huronensis and P. aquilonis were previously considered synonymous with P. hyperborea.

| Pollinator versus visitor
We retrieved >500 records of Platanthera-insect associations (Table 2).There were 154 associations among orchids and insects that were specific in stating it was or was not a true pollinator relationship.One association for P. hologlottis had contradictory evidence from two different studies (see records for Thysanoplusia intermixta in Table 2).One study (Dobson, 1988) cited wrong information from an original source -P.dilatata was referenced even though this species was not studied by the original authors (Thien & Utech, 1970).These 154 pollinator accounts span 30 (19.11%) species of Platanthera.Seven records state an insect Order as the definitive pollinator, usually citing other papers, but not necessarily the original source, for their information.Two instances, both for P. lacera, clearly referred to an insect as a visitor rather than pollinator.Only 92 (59.74%) of the 154 pollinator associations provide additional information pertaining to the time of day (e.g., nocturnal vs. diurnal) during which the interaction was observed, and even fewer provided information about insect behavior and/or pollinia placement on the insect.
We retrieved 233 unique species of insects interacting with 30 bird genus was also noted interacting with P. ciliaris and P. orbiculatata.At the level of Order, Lepidoptera were associated with 43 (27.39%)orchid species, while Hymenoptera and Diptera were associated with 12 (7.64%)and nine (5.73%) orchid species, respectively.
From the Lepidoptera, the families Noctuidae, Sphingidae, and Geometridae interact with high numbers of Platanthera spp.(22,17 and 13 species of orchid, respectively).On average, 9.00 (SE = ±1.62)insect species interact with each orchid but the average number of orchids for each insect species is 1.31 (SE = ±0.05),highlighting that each orchid can attract several insects but that relatively few insects are shared across orchid species.
Twenty-four insect associates of Platanthera spp.have been (Lepidoptera: Nymphalidae), have substantial conservation concerns despite being listed as Least Concern.Due to the lack of assessments for the vast majority of insect associates of the Platanthera -combined with increasing documentation of insect declines (Hallmann et al., 2017;Klink et al., 2020), particularly native pollinator declines (Young et al., 2017) -it is likely that conservation issues are not limited to a small handful of insect species.

| Specificity or generality of associations
Few Platanthera spp.were cited as being highly specific or only associating with one insect species.Just 15 (9.55%)Platanthera spp.had five or fewer insect species associated with them.In contrast, P. chlorantha had 41 (17.52% of all unique insect records) insect species associated as pollinators or visitors.The greatest Order-level diversity was associated with P. blephariglottis, P. integrilabia, and P. stricta -each with four (80.00% of the total insect Orders) associated insect Orders.When considering Family-level associations, P. dilatata (11 or 36.67% of all insect Families) and P. stricta (10 or 33.33% of all insect Families) had the greater diversity in pollinators/visitors.In total, 20 orchids shared visitors or pollinators, and 43 insects shared at least two Platanthera spp.
Two sets of six Platanthera spp.(P.blephariglottis, P. ciliaris, P. dilatata, P. grandiflora, P. integrilabia, P. peramoena and P. blephariglottis, P. ciliaris, P. chapmannii, P. grandiflora, P. integrilabia, and P. peramoena) were associated with the same visitor or pollinator species (Papilio glaucus and Papilio troilus, respectively), representing the greatest level of observed pollinator sharing.Figure 4 shows the insect Family-level associations, and degree of floral visitor and pollinator sharing for the 10 Platanthera spp. with the greatest number of insect observations.The average pollinator sharing index was 0.002 across all orchid species.Five animal Orders were associated with Platanthera spp.; one of which was Apodiformes (hummingbirds).Diptera, Hymenoptera, and Lepidoptera were associated with Platanthera of all spur length categories.Coleoptera were associated with all spur lengths except the medium category.Twenty-four insect Families were associated with short-spurred orchids, 19 with medium spurs, 18 with long spurs, and nine with very long spurs (Figure 5a).Trochilidae (hummingbirds) were only associated with the medium and long spur categories.Seven Families were associated with all spur length categories (Apidae, Geometridae, Hesperiidae, Noctuidae, Papilionidae, Sphingidae, and Syrphidae).Notodontidae, Oedemeridae, Proxidae, Staphylinidae, and Tortricidae were only associated with shortspurred Platanthera spp.
With respect to flower color, we found no pattern at the Ordinal level -every Order of visitor or pollinator was associated with every flower color category.At the Family level, 23 were associated with white flowers, 20 with green flowers, and 22 with other colored flowers (Figure 5b).A total of 13 Families were associated with all flower colors.Four Families (Mordellidae, Notodontidae, Plusiinae, and Vespidae) were associated with white flowers only.However, many of these data points represent single observations.Sphingid and noctuid moths were most commonly associated with white (12 orchid species each) and other colored Platanthera spp.(eight orchid species each), while noctuid and geometrid moths were commonly associated with green flowers (16 and 11 orchid species, respectively).

| Diurnal versus nocturnal interactions
The time of insect interaction with flowers was available for 122 insects (52.14% of all insect spp.) spanning 36 (22.93%)Platanthera spp.(Table 2).All 36 of those Platanthera spp.are described as having nocturnal pollination/visitation, 32 (20.38%) had diurnal pollination/ visitation, and 10 (6.37%) had both diurnal and nocturnal pollination/ visitation (Figure 3b).It is important to note that several Platanthera are included in multiple categories because some authors found differences in insect activity in different regions or years.Few literature sources (36/122 useful sources = 29.51%)account for the current knowledge of Platanthera-insect interaction timing, and many of those simply cite previous studies.
Nocturnal interactions were reported more frequently than diurnal interactions for all color categories.Green-flowered species appeared to have more nocturnal interactions (61.1% of all nocturnal interactions), compared to other colors, but these data were not significant according to a Fisher exact test (white vs. green two-tailed p-value = .61;green vs. other two-tailed p-value = .42).Of the 27 Platanthera spp. reported as having a scent, nocturnal floral scent production (11/27 = 40.74%)was observed more frequently than diurnal scent production (5/27 = 18.52%), but true tests to assess scent production time were not performed.

| DISCUSS ION
To better understand the current state of knowledge relating to the 157 Platanthera spp., and their pollinators, we read 297 unique literature sources, of which 122 provided useful information.In spite of Platanthera being a widespread and species-rich genus, many Platanthera spp.lack basic distribution and other natural historical information, and relatively few species (19.11%) have floral visitor or pollinator information.While many literature sources seem to provide visitor/pollinator information, it is often information cited from an older study, with few new studies investigating the pollination ecology of Platanthera spp.Below, we discuss our results in more detail and provide some recommendations.

| General Platanthera information and trends
Approximately 50% of terrestrial orchids are considered at-risk of extinction (Swarts & Dixon, 2009).For Platanthera spp., 12.74% of species are given an IUCN status.This is likely an underestimate of the number of species of conservation concern as it does not take into account national-or local-level policies.In addition, many species, particularly those present in Asia, and hybrid species have not been studied enough for robust conservation recommendations to be made.North America is often cited as a centre of diversity for Platanthera spp.(Wettewa et al., 2020;Wettewa & Wallace, 2021).
However, from a continental distribution perspective, Asia appears to be the diversity hotspot.This may suggest that Platanthera sensu lato has Asian origins, or it may simply reflect differential rates of colonization, radiation, and extinction throughout a previously cosmopolitan distribution.Interestingly, all species with fringed labella were described as having North American distributions, and flower colors other than green or white were associated with North American and Asian distributions only.These observations add more weight to the ideas that there are differential rates of colonization, radiation, and extinction and suggest that continental patterns in insect diversity influence Platanthera spp.(e.g., Stebbins most efficient pollinator principle (Stebbins, 1970)).It is difficult to make solid statements about worldwide Platanthera diversity because of uneven sampling and reporting.For instance, there were no literature records of Platanthera spp. in South America, but there are observations attributed to Platanthera spp.there on public databases that need to be verified.In addition, sampling and reporting within continents (e.g., Africa) are often clustered so as to provide minimal information on true distributions.Flower morphology is often linked to pollination syndromes and levels of pollinator specificity (Gravendeel et al., 2004) even though attempts to group co-associated traits may represent an oversimplification (Ollerton et al., 2009).Spur length was often cited as a strong reproductive isolation mechanism in Platanthera spp.because particular insect taxa are better suited to acquire nectar from particular flower spurs given the length of their proboscis (Boberg et al., 2014;Chapurlat et al., 2015), although other floral features like distance between anthers can play a role (see "Specificity or generality of associations" in the discussion), and some authors suggest that spur length variation is driven largely by climate (Bateman & Sexton, 2008).Scent is another important cue that helps attract certain pollinators or other insect visitors.For example, the timing with which scent is released, and the chemical composition of the scent can lead to effective reproductive isolation among species and populations (Brand et al., 2020;Gaskett, 2011).Thus, we summarized morphological information using broadly defined categories but note that there is considerable variation among observations suggesting that these floral traits may be locally adapted to pollinators (Boberg et al., 2014).A high proportion of species were described as having short spurs (39.49%), adding support to the hypothesis that the ancestral form for Platanthera spp. was short spurred, and that spur length has been increasing over time (Hapeman & Inoue, 1997).
A high number of species were identified as emitting a scent at night, again supporting the idea that short-tongued nocturnal moths were the likely pollinators of the ancestral Platanthera form (Hapeman & Inoue, 1997).However, the knowledge gaps for a considerable number of species (e.g., 41.40% of species have no spur measurements; 80.89% of species have no reported scent information) make it difficult to conclude much with certainty.

| Pollinator versus visitor
Considering the ecological importance of pollination, it is surprising that there is such limited information for many plant-pollinator interactions in this group.There were accounts of plant-pollinator interactions for just 19.11% of known Platanthera spp.Nearly all of those interactions involved insects -just one bird species was observed.Many sources did not make a clear distinction between visitation and pollination.Several factors contribute to the challenge of classifying specific relationships, including the sheer numbers of orchid-insect species interactions, accurate identification of insects and orchids, geographic and temporal variation in interactions, the unreliable nature of flowering (i.e., terrestrial orchids can remain dormant for several years (Reddoch & Reddoch, 2007)), and the relatively short flowering times (i.e., flowers may last as little as 7 days (Stpiczynska, 2003)).To successfully conserve orchid and associated insect populations, we must develop detailed databases of the known Platanthera-insect relationships across both time and space; pollination network studies may be more successful.
The distinction between visitor and pollinator typically requires that observers document: (1) transfer of pollen onto an insect, (2) subsequent movement of pollen by the insect, (3) transfer of pollen from the insect to the stigma (i.e., pollination), and (4) that pollination results in fertilization (i.e., pollen tube growth to the ovaries) (Cox & Knox, 1988).Complicating the distinction further, additional orchidspecific conditions were listed by van der Cingel (2001), including: (5) floral rewards must be shown to be consumed or otherwise used by the insect, (6) variation in floral morphology must be demonstrated to attract the insect, (7) contributions of pollen versus ovules to the next generation must be the result of pollination processes, (8) inter-relations of multiple vectors must be demonstrated at the community level, and (9) in the case of pseudocopulation and shelter rewards, sexual stimulation by scent, and sleeping in flowers must be demonstrated, respectively.Theoretically, this list of conditions provides a robust framework for distinguishing anything from casual visitors to true pollinators.However, the practicality of obtaining data to meet these conditions is often a limiting factor.
Many authors continue to equate floral visitors with true pollinators (e.g., Steen and Mundal (2013)), or equate pollen transfer and movement by a vector as definitive orchid pollination (e.g., Esposito et al., 2017;Smith & Snow, 1976), without confirming the other necessary conditions.While these are convenient proxies, without experimentation directed at proxy accuracy, the number of true pollinators for each Platanthera spp.may be inflated.For instance, Hattori et al. (2020) photographed floral visitors and then inferred fertilization rates from pollinia removal and capsule development among non-bagged and bagged-plants, but this is not a direct link to a specific pollinator and its efficacy in pollination or fertilization (or even abiotic factors).The current bottlenecks for Platanthera spp.
pollination studies appear to be in observing pollinia removal and deposition, linking pollination with fertilization, and demonstrating that rewards and morphology influence pollinators.
Additional authors have hypothesized that only long-tongued insects would be able to obtain nectar rewards from species with longer floral spurs (Bateman et al., 2013;Chupp et al., 2015).Our literature review confirms that noctuid, sphingid, and geometrid moths are the most common Lepidoptera associated with Platanthera spp., irrespective of spur length.
The state of knowledge for Platanthera pollination ecology is low compared to some other groups of orchids.For instance, a review of Euro-Mediterranean orchids revealed that 243 of 512 (47.46% vs. the 19.11% reported here for Platanthera spp.) known orchid species in the region have known pollinators, and that 773 insect species interact with those 243 orchids, representing an average of 3.18 insects per orchid species (Schatz et al., 2020).
That review identified 87 interacting insect Families across four Orders (Schatz et al., 2020).Our review covered a much broader geographic range, which may explain why we found slightly more insect Orders (i.e., five).Although we considered fewer orchid (259) and insect (233) species, the average number of insects per orchid species (9.00) was found to be considerably higher than that observed for orchids in multiple genera across the Euro-Mediterranean (3.18).The pollinator sharing index calculated here (0.002) was two orders of magnitude lower than the 0.46 value reported for Platanthera spp.by Schiestl and Schlüter (2009).These results indicate that Platanthera spp.generally receive a more diverse assemblage of visitor or pollinator partners in comparison to some other orchid groups, but the number of shared pollinators is low suggesting that effective reproductive isolation may exist among some sympatric species.Twenty Platanthera spp.shared visitors or pollinators, with two sets of six Platanthera spp.sharing a single insect visitor or pollinator (Papilio glaucus and Papilio troilus).Linkages such as these may be increasingly important to identify.Identification of shared pollinators will allow for more successful reintroductions because informed pollinator surveys will allow predictions of successful reproduction, although mycorrhizal symbionts (Wright et al., 2015) unintended hybridization events would also need to be considered.
Many authors have attempted to identify suites of floral traits that can be used to generalize or predict the pollinators of certain flowering plants (Fenster et al., 2004;Vereecken et al., 2010).

Sources advocating for predictive pollination syndromes indicate
that spur length, flower color, and flower size are the most important traits impacting pollinator behavior (Bateman et al., 2013;Hapeman & Inoue, 1997;Schiestl & Schlüter, 2009).For example, the orange flowers and long spur of P. ciliaris were expected to be pollinated by diurnal, long-tongued butterflies (Chupp et al., 2015).Catling and Catling (1991) predicted that white flowered species with long spurs, fringed labella, and scent released at night should be pollinated by large moths, while green flowers with short spurs should be pollinated by other Lepidoptera and other groups.Such predictions provide a starting point for further experimental work but provide little useful information in the interim as both morphological groups are effectively pollinated by Lepidoptera.
Additionally, work on P. lacera shows that spur length variation is not under selection from its primary pollinator (Little et al., 2005).
The findings from our review contradict some long-held beliefs about Platanthera pollination syndromes -specifically the assertions that green-flowered plants are primarily pollinated by pyralid and noctuid moths; white-flowered plants by noctuid and sphingid moths (Hapeman & Inoue, 1997).We found that geometrid and noctuid moths are most commonly associated with green flowers, and that while noctuids and sphingids (both are known to associate with eight Platanthera spp.) are often associated with white flowers, they are also commonly associated with non-white flowers.Platanthera grandiflora is presented as having a pollination syndrome suited to diurnal sphingids (Hapeman & Inoue, 1997), whereas our review shows that Papilio spp.(Papilionidae) are more often associated with its flowers.Further, other work states that scent can be a more powerful cue than visual stimuli for some insect groups (Inoue, 1985).
Such examples serve as a good reminder that while pollination syndromes may be useful generalizations, they should not be automatically equated as representing the full diversity or complexity of possible relationships.
Pollination syndromes are also confounded by spatial and temporal variation.We found considerable variation in reported spur lengths.For example, P. chlorantha spurs were reported as 20 mm (Maad & Nilsson, 2004) and as 32 mm (Stpiczynska, 2001).
Such differences suggest that regional and temporal climatic conditions, genetic diversity, and local pollinator abundances may drive local selection in spur length.Some populations may experience different selection pressures depending on what their local pollinator is, its relative efficacy in transporting pollen, and its local density.Indeed, some authors have noted substantial ecotype variation in response to the longest-tongued insect in the area (Robertson & Wyatt, 1990).In contrast, a study of Eurasian Platanthera spp.suggested that spur length may be better explained by latitudinal gradient and environment rather than by pollinators (Bateman et al., 2013), and a Scandinavian study showed that spur length variation was explained solely by abiotic conditions (Boberg et al., 2014).The low numbers of Platanthera spp. in longer spur length categories suggest that shorter spurs are likely to have been the ancestral state for this character, as indicated by Hapeman and Inoue (1997).
The ancestral placement of pollinia on vectors is believed to be the tongue or proboscis, with more-derived Platanthera spp.attaching pollinia to the eyes of pollinators (Hapeman & Inoue, 1997;Maad & Nilsson, 2004).Just 26.75% Platanthera spp.
had descriptions of where the pollinia were placed on the body of the vector -all of which were concentrated around the head.This is an expected pattern considering the structure of the flowers; vectors must insert mouthparts toward the spur at the back of the flower in order to obtain nectar.Slight differences in column structure are believed to drive specificity and reproductive isolation in many orchid species (Bateman et al., 2013;Catling & Catling, 1991;Schiestl & Schlüter, 2009).Indeed, subtle changes in column width are believed to be primary mechanism for increasing reproductive isolation among P. chlorantha and P. bifolia (Schiestl & Schlüter, 2009) and P. psycodes and P. grandiflora (Stoutamire, 1974).In this context, it is surprising that we found 29 insect species interacting with P. chlorantha, all with pollinia placement on the eyes.In contrast, Schiestl and Schlüter (2009) suggest that increasing pollinator specialization may simply be linked to orchid population density -smaller populations need to ensure maximal pollination rates so they tend toward increasingly specialized relationships.These data highlight considerable knowledge gaps as the specifics of many Platanthera-insect relationships remain unknown.
Much of the data presented here appear to reflect bias in the Platanthera species studied.For example, P. bifolia and P. chlorantha appear to be very popular research subjects, presumably because many researchers have been interested in understanding the hybridization events that occur between them.Thus, the specific research questions being addressed will clearly influence how much plant-insect data will be collected in any given study.
Underestimates of visitor or pollinator numbers are likely for difficult-to-identify groups, such as microlepidoptera and small Diptera.For example, a review of moth-orchid interactions across North America revealed 227 unique interactions representing just 129 species from seven Families -microlepidoptera, as a group, accounted for 25% of these interactions but there were few identifications to lower taxonomic (Hahn & Brühl, 2016).Further, few studies incorporate nocturnal investigations, and potential pollinator data are often recorded opportunistically rather than being a formal component of the study.

| Diurnal versus nocturnal interactions
Few sources specifically described the timing of the insect interaction.Those studies that mentioned observation time indicated nocturnal activity.Interestingly, few Platanthera spp.are expected to be diurnally pollinated as it is believed that the ancestral flower color is green, which is said to attract nocturnal moths (Hapeman & Inoue, 1997).Contrary to the expected patterns based on pollination syndrome theory, we found the reported numbers of Platanthera spp. with nocturnal versus diurnal interaction to be similar.The number of nocturnal interactions was greater than diurnal interactions across all flower color categories, although green-flowered species had slightly higher (nonsignificant) nocturnal interactions compared to white and non-white flowers.
Other authors have commented that Platanthera spp. with "other" colored flowers and fringed labella tend to be diurnally pollinated (Catling & Catling, 1991).
Nocturnal pollination has been hypothesized as facilitating highly specialized plant-pollinator relationships and subsequent reproductive isolation (e.g., one moth species to one orchid species) (Macgregor & Scott-Brown, 2020).A recent review of nocturnal pollination found that highly specialized pollinator-plant relationships are rare, primarily because most moths appear to be generalist nectivores (Macgregor & Scott-Brown, 2020).
However, many authors argue that even when orchids are making use of nonspecific pollinators, reproductive isolation may still be strong owing to differential placement of pollinia on the body of the pollinator (Catling & Catling, 1991;Efimov, 2016;Esposito et al., 2017).Several sister taxa appear to be functionally isolated due to differential pollinia placement on a shared pollinator (e.g., P. bifolia and P. chlorantha, P. leucophaea, and P. praeclara) in some parts of their range but not others (e.g., reports of P. bifolia and P. chlorantha hybrids) (Efimov, 2016;Nilsson, 1983aNilsson, , 1983b)).Thus, the link between nocturnal pollination and increasing plant-insect specialization seems weak.However, with limited research across Platanthera spp.and their ranges, it is difficult to make conclusive statements.
It is important to address the knowledge gaps relating to known pollinators and the timing of their activity.The management of orchids and their habitat should include best practices for managing a diversity of pollinators.For example, in a study on P. bifolia and P. chlorantha pollinator moth diversity was higher in taller vegetation but the orchids preferred shorter and less dense vegetation (Mõtlep et al., 2018).Some insect species appear to be experiencing local declines (e.g., Wagner et al., 2021), and many moth species in particular experience temporal and spatial variation in population density (Hahn & Brühl, 2016).In North America alone, there have been declines in eight species of Sphingidae and Saturnidae attributed to light pollution, forest clearing, and increases in parasitoid populations (Young et al., 2017).
Understanding the types of pollinators interacting with certain orchid species will improve management of orchid demographics and genetic diversity.For instance, there can be variation in the dispersal and foraging behaviors between diurnal and nocturnal insect species.The diurnal Aporia crategi (Lepidoptera, Pieridae) was observed moving greater distances than the nocturnal Zygaena minos (Lepidoptera, Zygaenidae), both of which pollinate Anacamptis pyramidalis (Orchidaceae, Orchidoideae) (Vereecken et al., 2010).Thus, diurnal and nocturnal insects may contribute to pollinia dispersal, and subsequent gene flow, in different ways which may also impact the relative rates of selection on floral traits.Knowledge of the behavioral and ecological differences between and among diurnal and nocturnal pollinators will further our understanding of, and management for, pollinator limitation -a common concern for many orchid populations (Chupp et al., 2015;Young et al., 2017).
Finally, to appreciate the intrinsic value of biodiversity (Rea & Munns Jr, 2017) (Gomulkiewicz et al., 2000;Thompson, 1999Thompson, , 2005) ) suggests that plant species with adjacent or overlapping ranges that interact with a suite of pollinators -as is the case for many Platanthera spp.-will have different and complex co-evolutionary outcomes.This is made more complex by the fact that many pollinators are generalists that are co-evolving with other non-Platanthera spp.Thus, evolutionary processes acting on Platanthera-insect interactions are the result of both co-evolutionary processes at that site and potentially sites and species much further away.Studying Platanthera spp.pollination in the context of co-evolutionary geographical mosaics (e.g., Johnson & Anderson, 2010) would shed further light on this concept and inform plant and insect conservation efforts.

| Linking pollinator needs and behavior to orchid reproductive success -future research
Identifying co-evolutionary selective pressures among plants and insects is important but challenging.Botanists and entomologists often use different approaches -botanists may focus on the impacts of pollination on reproductive success (Inoue, 1986;Vojtkó et al., 2015), gene flow (Wallace & Bowles, 2023), and population dynamics (Berry & Cleavitt, 2021), whereas entomologists may focus on the energetics of obtaining a reward or trying to fly with a pollen load, synchrony among plant and pollinator phenology, or attempts to cheat the system through nectar robbing (Feuerbacher et al., 2003;Fox et al., 2015;Mountcastle et al., 2015;Slominski & Burkle, 2021).Efforts to study both the behavior and energetic needs of pollinators and the subsequent consequences on reproductive output in plants, simultaneously are not always common.Yet it is clear that disruption to mutualistic relationships among plants and animals can have extreme impacts on population dynamics, local extinction rates, and other evolutionary processes (Chupp et al., 2015;Gaskett, 2011).More integrated approaches that consider the interaction from all directions are needed.(Stpiczynska, 2001(Stpiczynska, , 2003)).Nectar availability was lower in younger flowers (Stpiczynska, 2001(Stpiczynska, , 2003)).
Similar patterns of flower maturation in P. stricta were observed by Patt et al. (1989) with 70-90% of flowers opening acropetally within 2 weeks.In P. praeclara, nectar levels and sugar concentrations gradually decline over the flowering period, but sugar concentrations are typically higher in flowers lower on the inflorescence (Westwood et al., 2011).Pollinated flowers continue to provide nectar for several days (Stpiczynska, 2003).These patterns suggest resource allocation trade-offs in which older flowers, which may be pollinated earlier, receive more investment and continue to attract pollinators so that younger, possibly less attractive and rewarding flowers, at the top of the inflorescence receive pollen too.
Few descriptions of insect behavior at or on the plants are provided in spite of the potential for this to influence pollination efficacy.Landing on the labellum is believed to be the ancestral pollination mechanism (Hapeman & Inoue, 1997).It is likely that hovering behaviors are more energetically expensive (Brzosko & Bajguz, 2019;Campos et al., 2015).If this is true, we might expect hovering behaviors to be associated with long-spurred flowers as they may provide greater quantities of nectar.However, of the Platanthera spp. with clear references to hovering behaviors by their pollinators, the literature indicates that they have variable spur lengths.Similarly, few studies have considered how weather can impact the behavior of insect pollinators, and the consequence such behavioral shifts may have on orchid reproductive success (e.g., Friesen & Westwood, 2013;Maad, 2000;Maad & Alexandersson, 2004).
Studies of nectar composition are rare even though nectar nutritional content could affect visitation length and pollinator behavior.For example, bees prefer high sugar concentrations (35%), whereas hawkmoths prefer lower concentrations (19%) (Brzosko & Bajguz, 2019).The few studies reporting sugar concentration or composition for Platanthera spp.show considerable variation -P.praeclara nectar is 25% (Fox et al., 2013;Westwood et al., 2011) and P. stricta is 8% (Patt et al., 1989) sugar.The concept of pollination syndromes suggests that P. stricta must be pollinated by something other than bees or hawkmoths given the low concentrations of sugar.However, several bees are cited as pollinators for this species (Patt et al., 1989).
There is a growing body of evidence of three-way interactions among pollination vectors, nectar microbiome, and fitness impacts on plants (Chappell & Fukami, 2018;Jacquemyn et al., 2013;Tsuji & Fukami, 2018).However, few studies have specifically investigated the chemical or microbial composition of Platanthera nectar, and few links are made among plant investment in nectar relative to pollinator specificity and subsequent pollination efficacy.Processes such as nectar microbes influencing the composition, quantity and volatiles of nectar, which have been identified in other plant species (Toju et al., 2018;Vannette & Fukami, 2018), may be occurring in Platanthera spp.and could reinforce or breakdown pollinator specificity, for example.For the few Platanthera spp.for which scent profiles have been developed, it is clear that species and populations exhibit considerable variation (Nilsson, 1983b;Steen et al., 2019), and there is evidence to suggest that feeding on nectar can stimulate chemical cues in P. chlorantha and P. bifolia to attract more insect pollinators (Nilsson, 1983b).These interactions are rich with opportunities for basic chemical ecological investigations that could result in actionable conservation recommendations.
Information on pollinator foraging and dispersal distances and on the sex of pollinators is also sparse in the literature.Because movement of pollinia represents a substantial contribution to gene flow, an understanding of pollinator foraging behavior and dispersal distances is important in understanding orchid population structure, demography, the relative contribution between male and female fitness, and the likelihood of translocation or reintroduction success.
For example, variation in habitat can change the primary pollinator resulting in different foraging distances (Vereecken et al., 2010).
Relative visitation rates between male and female visitors/pollinators may be an important gap to fill as differences among the sexes in foraging and dispersal distances are known for many insects (Smith et al., 2019).Differences in the average proboscis length between males and females have also been noted -males have a longer proboscis and may be more likely to engage in nectar robbing than actual pollination (Chupp et al., 2015).In such situations, the density and sex ratio of insect populations is an important conservation consideration.

| CON CLUS ION
In this review, we have provided the most recent and comprehensive information for plant-animal interactions within the genus Platanthera.While many studies mention associations among orchids and insects, relatively few studies have added new information on the natural history of those interactions.We also note that there is missing information for many species, while other species are commonly studied -but generally on small spatial scales.The application of pollination syndromes to describe and predict interactions among Platanthera spp.and insects is complex and some generalizations appear to hold merit while others do not.Overall, we advise against the use of pollination syndromes to characterize Platanthera spp.pollination.Over time, more data from more locations have shown that Platanthera spp.appear to be generalists in terms of their interactions with pollinators (so far known to be mostly insects) -each orchid species associating with several insect species and displaying considerable spatial and temporal variation.More studies should focus on understanding the diversity of associations, and their spatial and temporal patterns, because Platanthera spp.offer the opportunity to better understand the ecological and evolutionary mechanisms contributing to floral diversification and co-evolution -and for better conservation of plant and insect populations.

(
19.11% of total orchids) Platanthera spp.These 233 insects represented 138 genera across 30 different families and six orders.One F I G U R E 2 Venn diagram illustrating the continental-level distribution of Platanthera species.Venn diagrams of flower color (a); timing of floral visitor or pollinator visits (b); and floral spur length categories (c).
assessed and are listed by IUCN.Of those, 21 are listed as Least Concern; two, Bombus fervidus F. and Bombus occidentalis Greene (both Hymenoptera: Apidae) are listed as Vulnerable; and one, Psithyrus suckleyi (Greene) (Hymenoptera: Apidae) is listed as Critically Endangered.Others, such as Danaus plexippus L.
Venn diagrams showing the insect floral visitor and pollinator associations with floral spur length categories (a) and floral color (b).
and to understand the co-evolutionary relationships among plants and insects, we need to know what insects perform pollination services for what plants and how these patterns may shift over time and space.Pollinator-plant interactions can occur across substantial geographic ranges with differing evolutionary processes in different locations depending on the various biotic and abiotic factors that are present.The concept of a geographic mosaic of co-evolution While data are sparse, some general patterns are present from the literature.Many of the interactions involved the pollinators beginning by probing flowers at the bottom of the inflorescence and working toward the top.This behavior seems consistent with the limited studies of floral maturation and nectar production for Platanthera spp.where flowers open from the base of the inflorescence toward the top, and nectar production begins approximately 2 days before the flowers open
Abbreviations for the International Union for the Conservation of Nature (IUCN) conservation status are DD, data deficient; EN, endangered; LC, least concern; VU, vulnerable.Insect sex, where provided, is abbreviated as M, male and F, female.Full citation details for literature sources can be found in Appendix S1.Network showing the extent of sharing among the10 Platanthera species with the highest number of floral visitor and pollinator observations at the insect Family-level.Thicker lines indicate a greater number of observed insect species belonging to that Family.Platanthera hyperborea interactions from North America were included under P. huronensis because these two represent synonymsplants previously attributed to P. hyperborea in North America are now attributed to P. huronensis.
Patt et al. (1989)att (1985),Patt et al. (1989), Lahondère et al. (2020), van der Voort et al. (2022) Diptera Culicidae Aedes sp.Pterophoridae Oidaemetaphorus sp.Platyptillia sp.Pyralidae TA B L E 2 (Continued) commonly reported behaviors include hovering near and probing flowers and probing flowers from the bottom to the top of the inflorescence spike.A total of 42 (26.75%)Platanthera spp.had descriptions of where the pollinia were placed on the vector -all of which were concentrated around the head.The eyes and proboscis were the most commonly cited areas for pollinia placement on insect visitors/pollinators.Only records for P. chlorantha and P. obtusata provided information on insect pollinator or visitor sex.Of the Note: